This invention is relates to a method and apparatus for determining when the ink supply within an ink cartridge is low, and more particularly relates to a method and apparatus for determining when an ink cartridge used in a postage meter should be replaced.
Digital printing apparatus utilizing known ink jet printing techniques typically have a source of supply ink which is used by a printhead for printing on a recording medium. Replacement or replenishment of the ink supply is periodically required in order to ensure that continued satisfactory printing occurs. Previously, the determination as to when the ink supply should be replaced or replenished was usually made by the operator when the images being printed began to appear light or spotty. This simple visual procedure proved quite satisfactory in a majority of applications such as typewriters, word processors, and computer printers, because if a document of unsatisfactory print quality was produced, the ink supply could be replenished or changed and the document reprinted with little impact to the user. However, in printing devices used, for example, in connection with scientific equipment or in facsimile machines, the failure by the printing device to produce a readable image and the corresponding loss of data associated therewith could present a significant problem for the user.
U.S. Pat. No. 5,068,806 addresses the problem associated with printing devices where the loss of image data is unacceptable. This patent describes an apparatus which counts every individual ink dot that is ejected by the printhead in printing the image data. The apparatus keeps a running total of the number of ink dots ejected by the printhead during printing and continuously compares this total to a predetermined number of ink dots. In the event that the running total exceeds the predetermined number, a message is provided to the operator advising that the ink supply is low and should be replaced.
Additionally, it is known from U.S. Pat. No. 4,202,267 and 5,131,711 to utilize either optical sensors or conductive electrodes in an ink supply structure. Each of these devices determines the amount of ink remaining in the ink supply structure and provides an indication when the ink level reaches a predetermined low level such that the ink supply can be replaced.
Each of the above solutions for determining when to replace an ink supply, such as a commonly used disposable ink cartridge in ink jet printers, has serious limitations if applied to a device such as a postage meter. That is, a postage meter prints a postage indicia on a mailpiece as evidence that postage has been paid. Typically, the postage is accounted for in the meter prior to printing of the indicia. Therefore, in the event that an illegible indicia is printed, the postage meter user has been charged for an indicia that was not used. Accordingly, the method of visually determining when to replace the ink cartridge is unacceptable for postal applications.
With regard to the use of conductive electrodes and optical sensors, they are an expensive solution for determining when to replace an ink cartridge. Thus, in the small office/home office (SOHO) business environment where postage utilization is not high volume and a low cost postage meter is desired, these solutions are not practical.
As for the solution of counting the actual number of ink dots fired by the printhead during the printing of image data as a means of determining when to replace an ink cartridge, it is also inadequate in the postage meter environment. That is, it is inherent in the structure described in U.S. Pat. No. 5,068,806 that the volume of ink consumed in printing is significantly greater than the volume of ink consumed during routine printhead maintenance functions so that the ink used during the performance of maintenance functions is considered to be at noise level and is not accounted for in determining when to replace the ink supply. This would typically be the case in many printing devices where continuous large quantities of variable images are being printed on a regular basis such as in a computer printer or a word processor. In this situation, since extensive amounts of printing are being done on a regular basis, the printhead nozzles tend to remain unclogged due to the heavy printing activity itself. Thus, the known periodic maintenance actions of flushing and purging the printhead to ensure that the printhead nozzles do not become permanently blocked by debris or dried ink are not frequently required, and the ink used during such actions can be ignored in the ink dot count. A postage meter however, presents a very different printing environment particularly in the SOHO business arena where a user may only utilize the postage meter on a very infrequent basis. In this scenario, where the amount of actual printing over extended periods of time can be very small, it is necessary that much more extensive and frequent automatic maintenance actions be performed on the postage meter printhead, as compared to the high print volume applications discussed above, in order to ensure that the printhead nozzles are not clogged when called upon to print the indicia image. Moreover, since the indicia image is substantially a fixed image of a predetermined size, the total number of ink dots required to produce the image is significantly less than most general purpose printer applications where pages of material are being printed. Therefore, even in an environment where the meter is used regularly, the need for more frequent printhead maintenance actions is still required for the postage meter as compared to most printing applications. Accordingly, if a postage meter simply counted the ink dots deposited during printing of the indicia as the method for determining when to replace the ink cartridge, the cartridge would run out of ink well before a warning was given to the operator because the extensive amount of ink utilized in performing the required maintenance actions would not be accounted for.